This disclosure relates generally to ion-selective field effect transistors (ISFETs) on flexible substrates.
Throughout this disclosure, reference documents are identified by bracketed numbers. The reference corresponding to each bracketed number is identified herein. Each of these references is incorporated herein in its entirety by reference.
The ion-sensitive field effect transistor (ISFET) is well established as a pH sensitive biosensor, [5] and biochemical sensing is possible if a biological recognition material is immobilized on the ISFET gate-sensor surface. These devices are typically configured as large parallel arrays of individually addressed extended-gate ISFETs, and manufactured on silicon CMOS wafers. [12] While large-area ISFETs are desirable because of their large sensitive capture area, manufacturing them using a CMOS wafer fabrication process can become cost prohibitive. Commercial high volume thin film transistor (TFT) technology, used to manufacture large-area organic light emitting diode (OLED) and liquid crystal displays (LCD), offers a lower cost alternative to also produce large-area ISFET biosensors. [15] However, prior TFT-based biosensor development was limited to producing and characterizing ISFETs on rigid, fragile glass substrates. [15] [16] This can restrict the range of diagnostic applications in which the biosensor must come in direct contact with human tissue, or in direct contact with food or drink, where the ISFET biosensor may need to be conformable and/or shatterproof.
An Ion-Sensitive Field Effect Transistor (ISFET) is a pH sensor first introduced in 1970 by Bergveld [1]. Their use as a pH sensor has been extensive because of their small size, robust and low power consumption. ISFETs have seen their use as pH sensor for variety of applications such as environment monitoring [2], explosive detection [3] and for developing low cost medical devices [4][5]. Due to limitation of optical methods to detect DNA [6] [7] there has been a shift towards non-optical FET based sequencing [8] [9]. Additionally, antibodies could be immobilized on the ISFET for pathogen detection [10] [11].
In practice, these devices are typically configured as large parallel arrays of individually addressed ISFETs, and manufactured on silicon CMOS wafers [12]. However, the sensing array size for conventional ISFETs on silicon wafer substrates is ultimately constrained by a photolithographic stepper field size limited to approximately 1 cm2 per die. Also the commercial use of the ISFETs has been limited due to the drift in the threshold voltage, which could be mitigated using vertical field cycling [13], and the difficulties in packaging the ISFET on a silicon wafer [14]. One solution to these conventional silicon CMOS ISFET limitations is to apply thin film transistor (TFT) technology, presently in wide use to manufacture large liquid crystal displays, to produce ISFET-based biosensors [15]. This enables sensing arrays to be much larger in area than silicon substrate ISFETs. Leveraging the scaling advantages of traditional liquid crystal display (LCD) TFT display technology, which can now manufacture displays on Gen11 sized substrates that approach 10 m2, also offers the additional advantage of dramatically reducing the sensor cost to pennies per cm2, which is key for disposable applications. However, prior TFT-based biosensor development was limited to fabricating and characterizing ISFETs on rigid and fragile glass substrates [16]. This can create a problem in food industry or water-quality monitoring applications, where the use of materials that can shatter is strictly forbidden [17].
Consequently, considering such limitations of previous technological approaches, it would be desirable to have a system and method for making a flexible ISFET.